Xiaomi: after 800 charge cycles with HyperCharge, the battery has 80 percent capacity left – Tablets and phones – News

Lithium ion batteries that can be quickly charged and discharged have been around for much longer and are nothing new in themselves.

Take, for example, the former A123 cells that can be used for, among other things, Li-ion starter batteries, where >20C is possible without any problem and charging and discharging currents of thousands of Amperes can run with a battery the size of a lunch box, without any problem and with a life span of many years. You also see this in newer cars with a 48V on-board network and a relatively small (500Wh) li-ion battery that must be able to effectively recover braking energy and, for example, be charged and discharged with 20C or more.

This often involves lithium iron phosphate or sometimes NMC technology, so other li-ion variants than what we typically see in telephones, with “large electrodes”. Read: relatively much surface. This comes at the expense of capacity, there is a certain trade-off. With high-C optimized cells (including electrodes and surface optimization) and thermal management of the cells, you can also design systems for eg 20C in such a way that the cell temperature does not exceed 20 degrees above ambient and a very long lifespan can be guaranteed. That is why, in addition to optimized cells, plug-in hybrids and EVs usually also have thermal management (liquid cooling/heating).

The lifespan of a li-ion cell is a factor in which, in addition to the quality of electrolyte, the temperature also plays a very important role. If the internal resistance of a cell is relatively high, you will have a lot of heat development when charging quickly and this is at the expense of the lifespan. For that reason, in general the following applies: The faster you charge, the shorter the lifespan.

Up to now, telephones have been optimized for a lot of capacity per volume & weight, not for extremely high C values. With the 200W charging technology for such a small battery, I very much wonder to what extent this is at the expense of capacity. We are talking about a 4000mAh battery, I suspect that it is a lot bigger and heavier than a 4000mAh which can be charged with max 20W for example.

Suppose that “overhead” for 200W charging capability is 50% volume. Then you could also choose to choose a 6000mAh battery with “only” 20W charging instead of a 4000mAh that you can charge with 200W. I personally would rather have 50% more capacity than extremely fast charging. I have no idea whether that estimate of 50% is correct in this case, perhaps the number is a lot smaller and the optimum lies elsewhere.

In addition, you want to charge as efficiently as possible, because all the heat generated by your DC/DC charging electronics also heats up the battery. Assuming a cell voltage of 4V, at 200W there will be 50A (!). The heat that is generated is mainly determined by 2 factors:
1) Je charging electronics
2) The heating of the cell of its internal resistance.

For the first one: If that’s really 98% efficiency, that’s very good! But it still means 4W dissipation of losses (2% * 200W). If the internal resistance of the cell is very good that would be a comparable value, suppose that is also 4W loss at 200W loading. Then you burn 8W in the phone if you charge with 200W. You should not have a contact resistance of, for example, a dirty connector, because then you immediately have a fire.

And if you do not actively cool then you will quickly have a warming of 30 to 40 degrees delta T, even with a large telephone and many aluminum elements. I think that goes very wrong in a tropical area with ambient 40 degrees, then you become phone 70 to 80 degrees and then I don’t think you get 50 charge cycles to 80%. Apart from the fact that you can’t grab your phone then . So it seems to me that under those circumstances the charging speed drops to 20W or 30W.

In other words, I think it’s a nice test, which may also be correct under 100% ideal conditions and ambient 5 degrees, but under normal circumstances you only have a lot of overhead from that fast charging capability and you’d better just have more capacity. and keep your charging electronics (and connectors etc) manageable.

[Reactie gewijzigd door Timewa op 11 juni 2021 22:31]

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